An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. Each printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector consisting of an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the pressurization chamber in order to propel a droplet out of the orifice, or a piezoelectric device which changes the wall geometry of the chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other recording medium in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the recording medium is moved relative to the printhead.
A common type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a media advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a direction that is substantially perpendicular to the media advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the recording medium, the recording medium is advanced; the carriage direction of motion is reversed, and the image is formed swath by swath.
The ink supply on a carriage printer can be mounted on the carriage or off the carriage. For the case of ink supplies being mounted on the carriage, the ink tank can be permanently integrated with the printhead as a print cartridge so that the printhead needs to be replaced when the ink is depleted, or the ink tank can be detachably mounted to the printhead so that only the ink tank itself needs to be replaced when the ink is depleted. Carriage mounted ink tanks typically contain only enough ink for up to about several hundred prints. This is because the total mass of the carriage needs be limited so that accelerations of the carriage at each end of the travel do not result in large forces that can shake the printer back and forth. As a result, users of carriage printers need to replace carriage-mounted ink tanks periodically depending on their printing usage, typically several times per year. Consequently, the task of replacing a detachably mounted ink tank should be simple and reliable within the printer.
Inkjet ink includes a variety of volatile and nonvolatile components including pigments or dyes, humectants, image durability enhancers, and carriers or solvents. A key consideration in ink formulation and ink delivery is the ability to produce high quality images on the print medium. Image quality can be degraded if air bubbles block the small ink passageways from the ink supply to the array of drop ejectors. Such air bubbles can cause ejected drops to be misdirected from their intended flight paths, or to have a smaller drop volume than intended, or to fail to eject. Air bubbles can arise from a variety of sources. Air that enters the ink supply through a non-airtight enclosure can be dissolved in the ink, and subsequently be exsolved (i.e. come out of solution) from the ink in the printhead at an elevated operating temperature, for example. Air can also be ingested through the printhead nozzles. For a printhead having replaceable ink supplies, such as ink tanks, air can also enter the printhead when an ink tank is changed.
Commonly assigned U.S. patent application Ser. No. 12/614,481 discloses removal of air from the ink in a printhead, by applying reduced pressure (for example, using a bellows pump) to an air extraction device. An air passageway is provided between the air extraction device and the ink chambers of the printhead. An air permeable membrane is disposed at the top of each ink chamber, so that air can be transferred from the ink chamber to the air extraction device, but liquid ink cannot. In some instances, for example if the printer is moved from a horizontal orientation or jostled excessively, it is possible for liquid ink to slosh onto the air permeable membranes. If the liquid is not removed from the air permeable membrane, it can impede the effectiveness of air removal through the membrane.
U.S. Pat. No. 7,491,258 discloses a gas and liquid separation device for use with a fuel tank of an automobile. The gas and liquid separation device includes a membrane that is oriented at an angle with respect to the horizontal to facilitate draining of liquid off the membrane so that gaseous fuel is passed through the membrane effectively. It is indicated that preferably the membrane is oriented near vertical (i.e. substantially or approximately 90 degrees with respect to horizontal).
What is needed is an inkjet printhead assembly including an ink chamber having an ink outlet that is fluidly connected to an array of nozzles, and an air-permeable membrane that is positioned in the ink chamber to facilitate removal of liquid ink to keep it from adhering and blocking the membrane, as well as a compact design. For the case of a carriage printer, an orientation of the membrane is preferred that facilitates removal of liquid ink during carriage acceleration and deceleration.